There has been an explosive growth in wireless communication and the emergence of commercial and consumer applications of radio frequency (RF), microwave, and millimeter-wave circuits and systems in a number of areas. These areas include wireless personal communication and local area networks (WLAN), satellite communications, and automotive electronics. Future personal and ground communications systems as well as communications satellites impose requirements such as very low weight and low power consumption, and small volume. The decrease in size and weight, the ever increasing frequency as well as the trend toward greater functionality of the communications systems, platforms are necessitating the use of highly integrated RF front-end circuits. Continuing chip scaling has contributed a lot to this goal, at least to the boost in the functionality of the devices or to the increase in operational frequency of e.g. CMOS-based technologies. However today a situation has been reached where the presence of the expensive, off-chip passive RF components, whether tunable or not, such as high-Q capacitors, high-Q inductors, resistors, switches, varactor diodes, high-Q resonators, and filters plays a limiting role.
Printed circuit boards have been used extensively as a technology platform to which these individual electronic components are mounted. To provide greater densities and more flexibility, sets of chips can be mounted on a separate packaging substrate. Various designs of these substrates have been proposed, the major ones of which are described in the book “Multichip Module Technologies and Alternatives”, D. A. Doane and P. D. Franzon, Van Nostrand Reinhold, 1993. An example of such substrate is the Multi-chip module substrate (MCM substrate) that typically provides an interconnect facility or sometimes also integrated simple passive devices such as resistors.
To provide tunable components with movable parts, e.g. switches, traditional semiconductor processing has been modified to produce micrometer devices and has come to be known as Micro electromechanical Systems (MEMS). Generally, MEMS processing is non-conventional and separate rather than integrated devices have generally been produced.
To provide a complete RF device/system various separate components have to be brought together. A first approach is called the hybrid approach. This hybrid approach combines components, manufactured in various technologies, each having its own purpose, specifications and targets, on a single technology platform, adapted for receiving and interconnecting this variety of components. Any RF-MEMS variable Integrated Passive Devices (IPDs) or any active circuitry, e.g. BiCMOS, GaAs or CMOS, are flip-chip assembled onto the platform, e.g. a microwave MCM-D carrier substrate containing the interconnects and the fixed IPDs, such as resistors, operating in the RF and microwave regime.
Today there are three mainstream technologies that can be used as a technology platform in such hybrid approach:
Ceramics-based (thick-film) technology, e.g., Low Temperature Co-fired Ceramic (LTCC).
Thin-film based technology, e.g., Multi Chip Module (MCM) using Deposited thin films (MCM-D).
Technologies based on the extension and scaling down of printed circuit board (PCB) or printed wiring board (PWB) technologies, e.g., MCM-L (where the L stands for laminate) constructed of plastic laminate-based dielectric and copper conductors.
The latter based technologies, i.e. PCB or PWB based technologies, are most commonly used in low frequency digital applications, and are not very suited for RF and microwave applications. The other two technology platforms, such as LTCC and MCM-D, can be suited for RF and microwave applications. Of these, LTCC allows the integration of capacitors, resistors and inductors in a single ceramic or glass-ceramic body. This is achieved by combining low-firing ferrite, dielectric and conductive materials in a multilayer ceramic process with sintering temperatures around 850° C. MCM-D is a recently developed technology that is based on thin-film techniques as used in the semiconductor IC industry but applied using different materials. Hereby, thin film MCM devices are fabricated by a sequential deposition of thin conductor and dielectric layers on a substrate by, e.g., evaporation, sputtering, electroplating, chemical vapor deposition (CVD) and spin coating. The layers are structured with standard photolithographic and etching or selective deposition. The second approach is called the monolithic approach. In this monolithic approach these Integrated Passive Devices or passive components can be integrated in or on a semiconductor chip. However, despite many years of research, such on-chip passive components based on electronic solutions, implemented and/or integrated in various RF-IC (Radio Frequency integrated circuit) technologies including BiCMOS, SiGe and GaAs, did not result in components with the high-quality offered by discrete passive components and which are required by most wireless applications.
A comparison of the different microwave IPDs technologies, also including the RFIC technologies, as outlined above, is presented in Table 1.
TABLE 1Comparison of the different technologies for microwave IPDs.PerformanceMax. freq.Process-Tunable/MetalW ± ΔWOn-chipTech.CostQ-factor(GHz)controlSwitcheslevels(μm)RF ICsMCM-DCheapQL = 30-15050AverageNo>210 ± 1nosubstrate(glass)1-1.5USD/cm2RF-MEMSCheapQL = 4080averageYes<210 ± 1nosubstrateQC = 50(glass)(varicap)0.5-1USD/cm2LTCCGlass-QL > 4010poorNo>3100 ± 2 noceramic1-1.5USD/cm2III-VVeryQL < 1015 (FET)goodyes>4  1 ± 0.1yes(GaAs)expensive110substrate,(HEMT)30 USD/cm2BiCMOSExpensiveQL < 850goodyes>1  1 ± 0.1yessubst.(BiPSiGe)(HRS,SiGe), 10USD/cm2III-V +VeryQL < 10<50averageyes>410 ± 1YesMEMSexpensivesubstrate
Based on this table, it is concluded that LTCC although applicable for microwave Integrated Passive Devices (‘IPDs’) has certain drawbacks when it comes to size, density as indicated by the line width W, and maximum operating frequency. Further, the on-chip passive components based on RF-IC technologies (BiCMOS, SiGe or GaAs) do not offer the high-quality as required by most wireless applications e.g. the Q-factor of the inductors is typically below 10, whereas Q-factors exceeding 30 are desired. Compare this to MCM-D Q factors, which are around 50 for fixed devices, e.g. both the capacitors and the inductors. One reason for the worse RF-behavior is the lossy substrates used in standard processes. Quality substrates such as High Resistive Silicon (HR-Si), GaAs and SOI substrates, needed for the manufacturing of active devices, are only available at a higher cost. Even in case of high quality substrates the RF-performance can be low. Due to the limited dimensions of the conductor wiring interconnecting the active and fixed passive devices on the substrate dielectric losses can be high. Not only the RF-performance but also the technology cost is an important issue. The cost for MCM-D is around 1-1.5 USD/cm2 (for a 7-mask process). This should be compared to the cost of 10 USD/cm2 for a standard BiCMOS process and an even higher cost of 30 USD/cm2 for a GaAs process. It should be said though, that the latter two processes not only offer integrated passives but full integration with active circuitry as well. Development of passive devices in such technology is however expensive as such changes might affect the overall technology, including the active devices. The development of a monolithic process is a complex and time-consuming process. Due to the shortcomings of RFIC technologies, ever increasing pressure is placed on the need to develop technologies for the fabrication of “integrated passive devices (IPDs)” operating in the RF and microwave regime. The thin-film integrated technologies generally provide the level of precision, range of component values, performance and functional density at a reasonable cost, which makes these technologies suited for the fabrication of microwave IPDs, thus allowing a more integrated, smaller, and lighter implementation of a given RF function compared to the monolithic approach
IPDs for today's wireless communication systems not only encompass fixed parameter value components, but also variable parameter value components such as RF switches or varicaps. As Table 1 indicates, variable or tunable IPDs can be fabricated in several technologies. As elucidated earlier however, the RFIC technologies are not suited for fabricating high quality IPDs, which limits the field of application of these technologies. The potential of RF-MEMS for miniaturization and integration, makes MEMS technology into a leading technology for the realization of variable IPDs, e.g., filters, switches, capacitors, inductors, with the potential of fabricating tunable/switchable modules, e.g. adaptive matching networks. The introduction of tunability and switchability in RF communication front-ends opens a way to design innovative, re-configurable RF transceiver architectures, like multi-band transceivers, which will be needed in present and next-generation wireless communication systems. It is expected that RF-MEMS technology will solve some of the most troublesome problems related to the use of discrete passive tunable/switchable components. The technology can yield small, low weight and high performance tunable/switchable RF components to replace some of the bulky, expensive and unwanted discrete passive RF components. Basically these RF-MEMS components contain movable parts and/or suspended parts, e.g., suspended inductors or transmission lines. All these features as offered by RF-MEMS make this technology a very attractive choice for the fabrication of variable IPDs. In addition variable RF-MEMS passive components in some cases display superior performance characteristics as opposed to their semiconductor counterparts.
TABLE 2Comparison of typical performance characteristics of RF switch typesGaAs MMIC(MESFET type)PIN diodeRF-MEMS switchInsertion loss (@2 GHz)0.51dB0.6dB<0.2dBIsolation (@2 GHz)−25dB−50dB−35dBReturn loss (@2 GHz)−20dB−10dB−35dBMax. RF frequency10GHz10GHz80GHzSwitching timetens of nshundreds nsHundreds of nsRF power handling30dBm30dBm30dBm (1W)Actuation/bias voltage+5V1+5, −5V>12VStandby powertens of micro Wfew mW“zero”consumptionIP337dBm44dBm>66dBm2Die size (SPDT3 switch)1.1 × 1.1 × 0.7mm30.8 × 1.3 × 0.7mm32 × 1 × 0.7mm3Body size<3.5 × 3.5 × 2mm3<3.5 × 3.5 × 2mm3<4 × 2 × 2mm31TTL compatible bias voltage.2Beyond the limits of measurement equipment (IP3 extrapolated as better than 66 dBm).3Single Pole Double Throw, e.g., implemented as a toggle for antenna switching or T/R switching.
An example is presented in Table 2 in which the performance characteristics of different types of miniature RF switches are compared. The table clearly shows the outstanding performance of RF-MEMS switches in terms of insertion loss, power consumption and linearity. For instance, the insertion loss of RF-MEMS switches is typically around 0.2 dB in the range 1-10 GHz, whereas FET type switches exhibit an insertion loss around 1 dB in the same frequency regime.
Table 3 presents an indicative performance overview of the different technology platforms implementing RF-MEMS. MEMS processing and materials are closely linked to semiconductor processing and materials hence, in the monolithic approach the MEMS part is processed with the semiconductor substrate, e.g. on top of the semiconductor device. An example of such integration is given in <<Monolithic GaAs PHEMT MMIC's integrated with high performance MEMS Microrelays <<by E. A. Sovero et al. (IEEE MTT-S IMOC 1999 Proceedings, Brazil, p 257). Such processing has the disadvantage that a MEMS device consumes relatively large and thus expensive chip area. Further the process freedom of such monolithic integrated IPD is limited. As was already the case for the fixed passives already implemented in such fully integrated process, e.g. CMOS, the characteristics of the underlying active devices may not be changed by the subsequent processing of the passive devices. Such process restriction will limit the type and number of IPD feasible in this monolithic approach.
TABLE 3Comparison of the different technology approaches implementing RF-MEMS.Technologies forMax.Range ofDegree ofCost ofRF-MEMS andRFavailableInterconnectminiaturizationTunable RFTime toTechnologyfixed passivesFreq.passivesflexibility(size, weight)modulemarketlifeHybridlowwideHighlowmediumShortLong(LTCC, MCM-D)MonolithichighlimitedMediumhighhighLongShort(GaAsMMIC + RF-MEMS)
Although the RF-MEMS technology has clear advantages as explained above, the technology also displays drawbacks. For instance, the interconnection levels are very limited and so are the number and quality of fixed passives.
Hence, it can be said, that none of the aforementioned technology platforms offers a flexible, cost-effective solution for the fabrication of a wide range of fixed or tunable high quality RF and microwave IPDs. It is an object of the present invention to provide such a platform and a method of making it.